This invention relates generally to an acid-type galvanic primary cell with a zinc anode, a depolarizing cathode and a gel electrolyte, and in particular, to metal additives which are useful to suppress zinc corrosion and improve mechanical tensile strength.
The present invention primarily relates to acidic zinc/Mn0 .sub.2 cells (Leclanche cells) and zinc/oxygen cells. In such cells, the zinc electrode is generally implemented as a can or hollow cylinder which contains the remaining active cell components (e.g., a Mn0 .sub.2 /carbon black mixture pressed around a carbon rod conductor, as the cathode; an electrolytic paste; and a separator substrate). Less frequently, the electrode is implemented as a flat metal plate in a coated cell structure.
According to R. Huber, "Dry Batteries", page 29 (VARTA Handbook Series, Volume 2, 1972), the properties of the anode plate are dependent on its method of manufacture, and its alloying with different quantities of lead and cadmium (the lead content can amount to as much as 0.6%). The alloyed lead increases ductility during the rolling and extrusion process, and also provides a slightly inhibiting effect on zinc corrosion. The addition of cadmium improves the mechanical tensile strength of the zinc-anodes during the battery manufacturing process.
In using zinc metal is dissolved in the battery during current drain with increasing duration. However, aside from this "useful" dissolution, a slight autodissolution of the zinc occurs, especially in case of extended storage periods of the cell, and even more at increased temperature. This is attributable to the fact that although the reaction of zinc in acidic solution is kinetically inhibited, the hydrogen formation occurs at a potential that is too low to stop the dissolution. The gradual accumulation of hydrogen in the interior of the cell leads to an increase in pressure, which under unfavorable circumstances can result in cell deformation, or even in a loss of electrolyte. If this hydrogen formation and dissolution of the zinc anode in an unloaded condition are not suppressed, then an undesirable reduction in capacity also occurs.
The recognized method for suppressing zinc corrosion and hydrogen formation, through an increase in the hydrogen over voltage, is to add mercury to the zinc. However, because of this, the number of components in the zinc which are classified as toxic and harmful to the environment increases to three (i.e., mercury, lead and cadmium). Because of this, constant efforts have been made to achieve a substitution for at least the particularly poisonous metals Hg and Cd. For example, DE-AS 1,086,309 describes an electrode of refined zinc with an additive of indium metal, while DE-OS 3,229,703 describes the use of indium and gallium as metals alloyed with the zinc. JP-OS 60-170751 (Toshiba Denchi K.K.) describes the addition of Pb and Li to zinc, while JP-OS 60-32249 (Touhou Aen. K.K.) recommends a zinc alloy of at least 0.01% Ag and other metals. Finally, DE-OS 36 05 718 discloses a zinc anode of refined zinc with an alloying addition of up to 0.6% Pb.
Despite the multitude of familiar alloying combinations involving zinc electrodes, only some of which are mentioned here, the basic purpose remained to improve the above-mentioned primary cell with non-toxic additives, particularly insofar as the zinc electrode is concerned, which permit the cell to perform equally well to a conventional acidic primary cell.